Bias-stress-induced stretched-exponential time dependence of charge injection and trapping in amorphous thin-film transistors

The threshold voltage instabilities in nitride/oxide dual gate dielectric hydrogenated amorphous silicon (a-Si) thin-film transistors are investigated as a function of stress time, stress temperature, and stress bias. The obtained results are explained with a multiple trapping model rather than weak bond breaking model. In our model, the injected carriers from the a-Si channel first thermalize in a broad distribution of localized band-tail states located at the a-Si/aSiNx interface and in the a-SiNx transitional layer close to the interface, then move to deeper energies in amorphous silicon nitride at longer stress times, larger stress electric fields, or higher stress temperatures. The obtained bias-stress-temperature induced threshold voltage shifts are accurately modeled with a stretched-exponential stress time dependence where the stretched-exponent β cannot be related to the β=TST/T0 but rather to β≅T ST/T0*-β0 for T ST≤80°C; for TST≥80°C, the β is stress temperature independent. We have also found that β is stress gate bias independent.